This document provides a general technical overview of three common
types of client modems seen in the field. With a good insight into the hardware
issues with modems, you can adjust the client configuration to achieve improved
performance.

This document also provides brief descriptions of chipset vendors.
Refer to the appropriate modem manufacturer documentation for more
detail.

Modems consist of two major components:

A datapump that performs the basic
modulation/demodulation
tasks for which modems are named.

A controller that provides the identity for the
modem. The protocols for hardware error correction, hardware data compression,
and basic modulation protocols (for example, V.34, X2, or K56 Flex) exist in
the controller. A controller also interprets attention (AT) commands.

Many Internet Service Providers (ISPs) encounter user complaints about
unstable connections, low connect speeds and so on. These issues can be caused
by client-side, Telco or circuit, or network access server (NAS)-side
problems.

General modem and line operational quality is closely tied to many
factors such as:

The ability of the NAS modem to interoperate with the vast and
ever-shifting range of peer modems (of various quality) encountered in the
field.

The quality of the modems on the client side as well as on the NAS.
The quality of the circuit (end-to-end connection) between the client modem and
the NAS.

The number of analog to digital (A/D) conversions in the
circuit.

You can troubleshoot the circuit and NAS side to ensure that they are
functioning correctly. However, you must also have a good understanding of the
mix of client modems.

In a hardware modem, the modem handles LIU, DSP and CP functions.
Hardware modems have historically been the best modems performance-wise, and
also the most reliable type. Hardware modems can be external or internal. With
external modems, a physical cable (such as an RS-232 serial interface) connects
the computer to the modem. In internal hardware modems, the internal bus of the
computer handles this function.

The line interface unit (LIU) handles the electronic signaling
interface to the Public Switched Telephone Network (PSTN) Network. LIU also
encodes and decodes the analog waveform to and from the Pulse Code Modulation
(PCM) used in the PSTN.

The digital signal processor (DSP) handles the modulation and
demodulation (V.92/V.90, V.34, V.32bis and so on).

The Control Processor (CP) handles:

Error Correction (MNP4, LAP-M/V.42)

Data Compression (MNP5, V.42bis, V.44)

Command interface (AT-commands, V.25) used by the DTE to
communicate with the
modem.

External hardware modems generally have better diagnostic
functionalities for troubleshooting. This is partly because they are quite
independent from the computer to which you connect them. Even the least
expensive models have a built-in speaker that allows you to detect retrains
easily. Lines with increased delay correspond to periods when modems retrained
(due to the link quality problems), which is easy to understand (hear) with an
external modem, but are not very obvious otherwise.

This is a sample output of pings (from a Windows PC) over an unstable
modem connection:

Most external modems also have LEDs to indicate the status of the
connection to the computer, and the activity over the phone line (user data
being sent and received). More advanced models have LCDs, and allow you to
monitor more details dynamically (such as, the current receive and transmit
rates, line noise, error level, signal quality, SNR, compression effectiveness,
and so on), as the line condition and data traffic change over time. As a
bonus, if the external modem freezes (for example, due to a problem in its
firmware), it can be power-cycled without rebooting the computer.

Internal hardware modems usually do not have LEDs. Such modems can use
the computer sound card to play the train-up phase, and often rely on the
software of the computer to report any details (which makes the results less
independent and reliable). Some advantages of internal hardware modems are
lower price, and potentially faster data exchange with the computer.

In controllerless modems, CP logic is moved into the computer operating
system, while the LIU and DSP are performed on the modem hardware itself. This
design is good because the hardware DSP still handles the real-time modulation
work, while the computer can handle CPU or memory-intensive data compression
function. With good design the difference between hardware and controllerless
modems is practically unnoticeable. This is because the CPU performance loss on
error correction, and data compression are compensated by data moving more
efficiently (with fewer interrupts) between the DSP and the computer.

These controllerless modems can be just as reliable, and perform at
least as well as hardware modems. However, some disadvantages are:

They share the same limitations as internal hardware
modems.

Controllerless modems may fail to work with a non-Windows operating
system (OS).

If the OS experiences memory problems, data compression may be
severely affected.

In HSP modems, the modem itself only consists of the LIU. It shunts the
PCM-encoded waveform through the internal bus to the CPU of the host computer
emulating the DSP.

The HSP design can still be quite effective, if the computer runs an
operating system capable of real-time processing. However, most HSP modems are
used on computers running Microsoft Windows OS, which is a non-realtime OS.
Therefore, HSP modems on Windows computers are often unstable, and experience
performance issues, especially when the signal processing functionality
competes for CPU cycles with normal computer time-sensitive functions like
sound, video and disk drivers operation.

Clients with HSP modems can expect unstable connections as well as
performance issues, such as lower speeds, high error rates and so on. A service
provider with a high percentage of HSP client modems should expect a higher
number of user complaints.

Look at the firmware version to identify whether you use a 56K or V.90
capable modem. In most cases, V1.1 or later is K56Flex and V2.0.65 or later is
V.90. The version determines whether the modem is able to make K56Flex or V.90
connections.

Some modem vendors also use V2.0.xx code to integrate K56Flex firmware.
For example, Boca has a 2.0.13 K56Flex firmware where 2.0.65 is the V.90 code.
This information applies only to some Rockwell clients.

Here is a list of common Rockwell Original Equipment Manufacturer (OEM)
vendors:

Best Data

Boca

Compaq

Diamond

Dynalink

Hayes for some K56Flex models (Hayes is no longer in business)

Lasat

Microcom

Multitech for certain V.90/K56Flex models

Practical Peripheral

Zoom (Lucent/Rockwell)

If you are not sure whether your modem is a Rockwell modem, go to the
homepage of the vendor to see if the Rockwell label appears. For a list of all
modem vendors, see 56K.COM's
56K Modem
Manufacturers page.

Users with firmware earlier than 1.1 must upgrade to V.90 (V2.0.65 or
later). Firmware versions earlier than 1.1 do not connect at 56KFlex or V.90
and fall back to V.34. Code earlier than 1.1 is also called K56Plus, a
pre-K56Flex code that MICA does not support.

Open a terminal session, make a direct connection to the modem, and
type the AT or at
command. The modem must respond with an "OK" message.

Type these commands:

Lucent AT i1 through AT i11
AT i99 Xircom !--- Tells you if you have a Lucent chipset.ATi3!--- Displays firmware revision.ATi11!--- Displays current or last call rate and diagnostic information.

Note: With Windows 98, you cannot view data in
ATi11 after a Dial-Up Networking (DUN) session. Use
a terminal program (such as HyperTerminal) to place a call to see the valid
diagnostic data.

Here is an example:

XIRCOM: V2.04 (Venus Chipsets)
Paradise Wavecom: V 5.39 (Winmodem)

If you want a V.90 connection on a Lucent client modem, force the
S109 register. For example, for Lucent clients that run recent
code, V.90 is achievable if the client has K56Flex disabled or, for Win Modems,
S38=0. For Venus, S109=2.

If you cannot 56 K connections with the newest version, make sure you
have the latest firmware. Also, limit the upstream (tx) rate
(s37=14) to see if that makes a difference. If you do not get
a 56 K connection with the old firmware, and you still do not get the
connection with new firmware (after you try s38=0), your V.34
connect rate can be slightly lower with the newer firmware. In this case,
return to the older firmware version.

If you call a V.90-enabled server, but KFlex connects, add
s38=0 in extra settings to disable KFlex. With LT firmware
later than 5.12, you can tell whether the handshake attempts V.90. There was a
major change to V.90 firmware at 5.12 with the introduction of digital
impairment learning (DIL) or "level-learning."

These HSP modems offload both the Controller Process (CP) and the
Digital Signal Processor (DSP) functions to the PC. You must have a high speed
CPU (200Mhz or better) in order to use these types of modems. For more
information, see 56K.COM's
Beware Soft Modems
article.

In order to determine the type of impairment, call an X2 or
V.90-enabled server with a terminal program. After you receive a CONNECT, wait
15 seconds or so and disconnect the call. Then, enter the
ATY11 command. The modem responds with a list of
frequencies and the receive level of each frequency. Look at the difference
between the value reported for 3750 and 3300hz. If this difference is 25 or
more, you can infer that there is more than one analog-to-digital conversion or
other serious impairment. If the number is close to, but less than 25, you may
or may not get a 56 K connection. If you do, the 56 K connection is very poor.
A good value for this difference is lower than 18.

Additionally, if the level reported for 3750 is above 50 to 55, you can
infer a poor local loop that can prevent or result in poor 56 K performance.

Here is a sample of the ATY11 output on a
connection that does not have more than one analog-to-digital conversion:

Ambient Technologies produces modem telephony chipsets that
manufacturers of internal and external modems design into their products. The
CL-MD56XX chipset family is a software solution that you can upgrade. The X2
USRobotics technology provides the data rate. See your PC product
manufacturer's web site for drivers and support. For more information, see the
Ambient
Technologies site.